1. Field of the Invention
This invention relates to an ion exchange process for recovering uranium from carbonate-containing waters or waste effluent. The method is particularly useful for treating process water or effluent derived from a common procedure for converting uranium hexafluoride to uranium dioxide of a grade suitable for use as fuel for nuclear fission reactors, or from solution containing dissolved uranyl carbonate anions from uranium ore leaching operations.
2. Description of the Background Art
One conventional means of producing fission fuel grade uranium dioxide consists of a wet conversion procedure, comprising the steps or reactions of: (a) hydrolyzing gaseous uranium hexafluoride (UF.sub.6) with water to form water soluble uranyl fluoride (UO.sub.2 F.sub.2) and hydrogen fluoride; (b) introducing ammonia ions, such as by the addition of an excess of ammonium hydroxide, to cause the soluble uranyl fluoride to precipitate as insoluble ammonium diuranate ((NH.sub.4).sub.2 U.sub.2 O.sub.7); and, (c) upon separation of said insoluble precipitate from the water fraction, heating the ammonium diuranate to drive off entrained fluorides with the ammonia and thereby convert the diuranate to uranium dioxide (UO.sub.2).
This basic uranium conversion process is disclosed in detail in the prior art, for example U.S. Pat. Nos. 3,394,997 and 3,579,311, and the disclosures and contents of said patents are incorporated herein by reference.
The ammonium fluoride containing effluent or process water derived from the aforesaid common uranium wet conversion procedure nevertheless retains relatively high proportions of soluble contents. In their existing chemical state the retained soluble contents are not amenable to removal by typical mechanical separating means such as by filtering, centrifuging or settling and decanting, and other physical techniques. The soluble contents include very significant amounts of about 10 to 70 parts per million of costly uranium as soluble complex fluoride, hydroxide, and carbonate anions, and mixed complex anions. The retention of such substantial amounts of valuable solubles in the aqueous system of this uranium hexafluoride to uranium dioxide wet conversion process, including significant quantities of uranium, and the economics and/or safety factors associated therewith, are subjects documented in the art, for example U.S. Pat. Nos. 3,726,650 and 3,961,027. The disclosures and contents of these patents are accordingly also incorporated herein by reference.
As noted in U.S. Pat. No. 3,726,650, the soluble uranyl complexes including fluoride, hydroxide, and carbonate anions, and mixed complex ions formed within the ammonium fluoride-containing water of the chemical system in the foregoing wet uranium conversion procedure are not readily recoverable. The soluble uranyl anions, or complexes thereof, have typically been removed in economically effective amounts from solution with strong basic anionic exchange materials, and subsequently stripped and removed therefrom for recovery with a strong mineral acid such as nitric acid. Acid salt solutions such as nitrate salts have been found not to be practical for stripping and removing uranium complexes from such anion exchange materials because of non-quantitative or low recovery results. Thus, an acid medium or presence is needed to provide a sufficient quantity of uranium removal and recovery from the ion exchange material to render the system practical and economically feasible.
Carbon dioxide gas has a strong propensity for, and rapid absorption rate into basic water solutions. This affinity renders it impractical or not cost recoverable to undertake to prevent carbon dioxide absorption from the air into basic aqueous media functioning within large production scale systems comprising storage tanks, settling basins and the like liquid handling units, such as those generally associated with the filters or centrifuges, ion exchange columns and the like in the commercial manufacture of uranium dioxide fuel by the wet conversion procedure. Moreover, water absorbed carbon dioxide, even in the parts per million quantity ranges, readily combines with uranyl ions and forms mono-, di- and triuranyl carbonate complex ions. Carbonates such as are typically formed in the basic aqueous medium of a wet uranium conversion procedure, concentrate on anion exchange material used in conjunction with the recovery of soluble complex uranium anions.
Any acid passing through a body of ion exchange material with carbonates retained or dispersed therein from uranium bearing, carbonate-containing water undergoing treatment acts upon the carbonates to produce and release large volumes of carbon dioxide gas throughout the ion exchange material. Ion exchange materials are typically employed as a bed of resin beads or particles and the released carbon dioxide gas in such large quantities as encountered under common conditions and contents with the aforesaid uranium wet conversion process, disrupts the integrity and continuity of the bed or body by raising or expanding and churning the mass of particles. Also pockets or voids of residual carbon dioxide gas can be formed within the exchange material which are difficult to remove and provide uneven flow channels or partial by-pass routes therethrough. Moreover, carbon dioxide gas enters into or forms within individual units or particles of the ion exchange material such as resin beads or granules. Thus any expansion of the gas at its inception, or due to heat and/or pressure changes can fracture or rupture a substantial proportion of the exchange material particles into small fragments. When diminished in particle size and uniformity, the costly ion exchange materials or particles are susceptible to high loss rates from the vessel or the system by being entrained and swept away within the liquid stream or current of the operating system. Particle loss is especially high when the exchange material is undergoing the usual treatments and/or rinsing for each rejuvenation cycle, an operation that typically entails the reverse or back flowing of a liquid through the exchange material and system for one or more steps thereof including flushing away entrained fines and recharging or generating expended exchange material.
Moreover, released gas within the system builds up pressure in the confines of ion exchange containers or column which can cause inadvertent rupture of such vessels or connections therewith and thereby create a hazard for both personnel and equipment.